Preliminary studies in full term infants indicate that active Hering-Breuer reflexes in infants, and in particular the tonic component which shortens expiratory time (Te) as absolute lung volume decreases, defend lung volume in the face of a naturally flexible and highly compliant chest wall. Normal term infants do not breathe at relaxation volume (Vr) while lying supine but rather actively maintain and defend end-expiratory lung volume (EEV) above Vr by utilizing the aforementioned vagal timing reflexes in conjunction with active expiratory braking mechansims. It appears that babies brake early expiration with persistent inspiratory muscle activity and/or by limiting expiratory airflow with narrowing of the glottis. Then in late expiration, they inspire before the relaxed respiratory system can reach Vr. These neural mechansisms can be thought of as providing a natural level of positive end-expiratory pressure (PEEP) to the respiratory system or "intrinsic PEEP" that maintains and defends EEV above Vr until the chest wall stiffens with maturity. We propose to investigate the reflex control of expiratory airflow and duration in a group of normal term and eventually preterm newborns by studying their response to sudden changes in resistance to expiratory airflow. We will carefully study the effect of altered expiratory flow rate on Te, and hence EEV. We propose to study a group of babies recovering from pulmonary disease requiring PEEP while intubated to explore the role of braking in these infants before and after extubation. Our initial expectations were that premature infants, particularly with their extremely flexible and compliant chest walls, also depend on "intrinsic PEEP" to actively maintain and defend EEV until their chest walls stiffen during development. However, preliminary observations suggest that normal preterm infants lack effective "intrinsic PEEP" mechanisms since EEV was observed to coincide with Vr in these prematurely born infants. Therefore, we propose to carefully study a defined population of premature infants first, to confirm if they indeed lack the ability to actively maintain and defend lung volume; secondly to determine at what age and over what time course do they develop "intrinsic PEEP"; and lastly, to correlate these maturational changes with selected breathing and gas exchange parameters.